1. Field of the Invention
The preferred embodiments of the present invention relate, inter alia, to a four stroke internal combustion engine, and, more particularly, to a four stroke internal combustion engine configured to generate a swirl in a combustion chamber by exhaust gas (e.g., burned gas) or fresh air (e.g., only air or an air-fuel mixture).
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
Conventionally, four stroke internal combustion engines having an exhaust gas re-circulation (EGR) system are widely used. In this system, a part of exhaust gas is returned to a combustion chamber to moderate burning of air-fuel mixture and lower the maximum combustion temperature in the combustion chamber to thereby decrease emission of nitrogen oxides (NOx).
For example, an EGR system structured as described below is known (see, e.g., pages 4-5 and FIGS. 4-5 of Japanese Unexamined Laid-open Patent Application Publication No. H05-86992). Specifically, the EGR system includes an auxiliary exhaust valve provided at an auxiliary exhaust port communicated with a combustion chamber and a gas reserving chamber configured to reserve a part of exhaust gas (EGR gas) discharged through the auxiliary exhaust port. The EGR gas reserved in the gas reserving chamber is returned to the combustion chamber at a preset timing.
According to such a four stroke internal combustion engine having the conventional exhaust gas re-circulation (EGR) system described above, improvement of fuel consumption resulting from decrease of pumping loss can be attained. In recent years, however, further improvement of fuel consumption has been desired.
Also, the four stroke internal combustion engine having the conventional EGR system described above requires the auxiliary exhaust gas port and the auxiliary exhaust gas valve in addition to a main exhaust gas port and a main exhaust gas valve for discharging exhaust gas. This complicates the structure of the cylinder head part, resulting in, e.g., increased manufacturing costs.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.